The present invention relates to a novel method for the synthesis of new fluorescent dyes, and more specifically to a method for producing novel dyes which are ring-constrained indoline-based rhodamine class dyes. These compounds have absorption and emission spectra which make them particularly useful in certain dye laser applications.
An important intermediate necessary for the efficient synthesis of these dyes is 6-hydroxyindoline.
Known methods for the synthesis of 6-hydroxyindolines are circuitous (R. R. Hunt, R. L. Rickard, "A new synthesis of methoxyindoles," J. Chem. Soc. (C) 1966, 344; M. Julia, H. Gaston-Breton, "recherches en serie indolique. XVII.--Preparation de quelques indolines, indoles et typtamines oxygenesen positions -4 ou -6 par cyclisation &lt;arynique&gt; "Bull Soc. Chem. France 1966, 1335), and a known method for synthesizing the methyl derivative involves an expensive and dangerous hydride reaction (CIBA Ltd., "1-Alkyl-6-carbamoyloxyindolines" Brit. Patent 726,078 (Mar. 16, 1955). Yet another method, known as the Leimgruber-Batcho method, can be used for indole synthesis ((a) P. L. Feldman, H. Rapport, "Convenient synthesis of 6-methoxyindole and 6-methoxytryptophyl bromide," Synthesis 1988, 735; (b) A. D. Batcho, W. Leimgruber, Org. Synth. 1963, 63, 214) followed by a problematic and expensive reduction to the indoline (G. W. Gribble, J. H. Hoffman, Synthesis 1977, 859). Separately, 4-hydroxyindoline has been produced (H. Tanaka, Y. Murakami, T. Aizawa, S. Torii, "A facile synthesis of 4-hydroxy- and 4-aminoindoles through corresponding indolines," Bull. Chem. Soc. Jpn. 1989, 62, 3742).
Thus, it is an object of the invention to identify and efficiently synthesize new compounds which are indoline-based rhodamine class dyes having absorption and emission spectra appropriate for utility in dye lasers.
It is a further object of the invention to identify a simpler, safer and less expensive method of producing such dyes through the 6-hydroxyindoline intermediate.